Many digital radio systems employ a sophisticated arrangement of acknowledgements and retransmissions to enable the receiver to correctly reconstruct the information content of each received data block. This is especially the case in packet switched radio connections used to convey non-real time services, because the nonpredictable need for retransmissions is ill suited for the tight timing requirements of real time services. A retransmission means that in some way or another the receiver makes the transmitter aware of that a block of data was not received in a good enough shape, so the transmitter must transmit at least a part thereof again.
Retransmitting the same piece of data several times consumes transmission time and bandwidth, which are scarce in multiple user systems like cellular radio networks. Incremental redundancy in general means that the transmitter tries to find and transmit the smallest possible portion of the previously transmitted information that would be enough for the receiver to perform its task of reconstructing the information content. Many known incremental redundancy schemes rely on the fact that the first version of the transmitted data block is punctured, i.e. a predefined pattern of bits thereof are intentionally omitted at the transmission stage. If the conditions of reception are good, the receiver is able to use the remaining parts of the data block to bridge the punctured gaps. If the first decoding round is unsuccessful, the receiver asks the transmitter to additionally give a number of the originally punctured bits.
Also incremental redundancy arrangements have their drawbacks. Even if the limited retransmissions are much smaller in bit number than completely retransmitted blocks, they still consume radio resources. The concept of retransmissions, be they complete or partial, inherently introduces delay because the receiver must react to an unsuccessful decoding attempt and find a scheduled turn for asking for the retransmission, and the transmitter must receive and decode the request and react thereto as well as wait for the next suitable time instant for transmitting the requested additional bits.